1. Field of the Invention
The present invention relates to data transmission systems and, in particular, to a quotient phase-shift processor circuit for realizing phase corrections of a digital phase-locked-loop.
2. Discussion of the Prior Art
A summary of digital phase-locked-loops is provided by William C. Lindsey and Chak Ming Chie in their publication entitled "A Survey of Digital Phase-Locked-Loops", Proceedings of the IEEE, Vol. 69, No. 4, April, 1981.
As stated by Lindsey and Chie, a classical analog phase-locked-loop consists of three primary functional units: a phase detector, a loop filter and a voltage controlled oscillator (VCO). The incoming signal to the phase-locked-loop is mixed with a locally generated sine wave in the phase detector to produce a voltage that is proportional to the instantaneous phase difference between the incoming signal and the local clock. This phase error voltage is filtered and used to control the instantaneous frequency of the VCO. All three components are realized with analog circuitry, e.g., the phase detector is a balanced mixer, the loop filter is an RC low-pass circuit, and the VCO is tuned by adjusting the bias voltage on a Varicap in the tuned circuit which controls the resonant frequency of the VCO.
Continued progress toward increasing performance, speed, reliability, together with the simultaneous reduction in size and cost of integrated circuits has resulted in continuing development of digital phase-locked-loops. In one type of digital phase-locked-loop, the phase detector determines at each cycle whether the input signal phase is early or late. Thus, the sampling phase detector provides a simple binary output which indicates whether the locally generated reference clock leads or lags the input signal. Because of the harsh quantization resulting from this approach, a sequential filter is often used to smooth the correction voltage applied to step the local digital controlled oscillator. The name "sequential filter" implies that the output is not a linear function of a fixed number of inputs. Instead, the sequential filter observes the inputs for a variable duration of time and provides an output when a certain confidence limit on the input is established.
The digital equivalent of the analog integrating element (such as an RC filter) is a digital accumulator. The digital accumulator is mechanized using an adder and a unit delay. By employed two accumulators in tandem, a digital approximation to a double integrator can be obtained. The construction extends, obviously, to higher order approximations.
A digitally controlled oscillator (DCO) is basically a programmable divide by N circuit. The output of a stable oscillator, usually at a frequency M times the IF frequency, where M determines the phase error resolution, is used to drive a counter which increases by one every clock cycle. The content of the counter is compared with the input, and when the two are matched, the comparator provides a pulse which serves as the output of the digitally controlled oscillator and to reset the counter. By varying the control input N, the DCO period can be controlled.